The present invention relates to an amorphous carbon covered member used for machine parts, molds, cutting tools, sliding parts, etc. to improve wear resistance, sliding properties and surface protective function.
Amorphous carbon films are carbon films or hydrogenated carbon films which are amorphous and also called diamond-like carbon (DLC), carbon hard films, a-C, a-C:H, or i-C. Since amorphous carbon films have excellent characteristics such as high hardness, high plane evenness and low friction coefficient, application to machine parts, molds, cutting tools, sliding parts, etc. for which wear resistance and low friction coefficient are required is expected. They are actually used for some of them.
As methods of forming amorphous carbon films, plasma CVD using a hydrocarbon gas such as CH4, sputter deposition, ion plating, vacuum arc deposition, etc. are used. But since adhesion between the substrate and the amorphous carbon film is poor, various methods for improving adhesion have been proposed. As a general way of improving adhesion of amorphous carbon films, forming an interlayer of various structures between the substrate and the amorphous carbon film have heretofore been tried. For example, in Japanese patent publication 64-79372, a method is disclosed in which after a 50-1000 nm thick interlayer of titanium carbide has been formed on a substrate by vapor phase synthesis, an amorphous carbon film is formed by vapor phase synthesis.
Also, Japanese patent publication 5-82472 discloses a structure in which an interlayer 0.1-10 xcexcm thick comprising at least one of carbides, carbonitrides, carbooxides, carbooxinitrides, carboborides of metals in the IVa, Va and VIa groups in the periodic table, carbides or carbonitrides of Si, or mutual solid solutions thereof is formed on a sintered alloy comprising at least one of carbides, nitrides or mutual solid solutions of metals in the IVa, Va and Via groups in the periodic table, and an amorphous carbon film is formed thereon.
Heretofore, the thickness of the interlayer was usually 50 nm or over. If an amorphous carbon film was formed on such a thick interlayer, adhesion was insufficient for machine parts, cutting tools and molds that are used under extremely high contact pressure. Fields were limited to which an amorphous carbon film was applicable.
After studying various structures to improve adhesion of an amorphous carbon film, it has been found out by the applicant that the following structure makes it possible to realize an amorphous carbon covered member which has such a high adhesion as to be applicable to machine parts, cutting tools, molds, etc.
Specifically, it has been found out that by forming on a substrate an interlayer comprising at least one element selected from the group consisting of elements in the IVa, Va, VIa and IIIb groups in the periodic table and elements in the IVb group except carbon, or carbides of at least one element selected from the abovesaid groups, and forming on the interlayer an amorphous carbon film so that the thickness of the interlayer will be 0.5 nm or over and less than 10 nm, it is possible to markedly improve the adhesion of the amorphous carbon film to the substrate.
As a structure for obtaining an amorphous carbon covered member that is superior in the adhesion to the substrate, the present invention is characterized by the material, film thickness and forming method of the interlayer.
As the material for the interlayer, at least one element selected from the group consisting of elements in the IVa, Va, VIa and IIIb groups in the periodic table and elements in the IVb group except carbon can be used. Since these elements react with carbon and form carbides, by forming an amorphous carbon film on the interlayer comprising one of these elements, a bond of such an element and carbon is formed at the interface between the interlayer and the amorphous carbon film, so that a high adhesion is achieved.
Otherwise, carbides of these elements may be used as the material for the interlayer. By forming an amorphous carbon film on the interlayer of such a carbide, a bond between the carbon in the carbide and the carbon in the amorphous carbon, or a bond between one of the elements in the IVa, Va, VIa and IIIb groups in the periodic table or one of the elements in the IVb group except carbon and the carbon in the amorphous carbon is formed at the interface between the interlayer and the amorphous carbon film, so that a high adhesion is obtained. These carbides may be of a composition within or out of a stoichiometric ratio.
Among these materials, it is especially preferable to use for the interlayer at least one element selected from the group consisting of Ti, Zr, Hf, v, Nb, Ta, Cr, Mo, W and Si, or a carbide of at least one element selected from the group. Since these elements are substances that can easily form carbides, by forming an amorphous carbon film on an interlayer of one of these elements or a carbide of one of these elements, a stable and rigid bond is formed at the interface between the interlayer and the amorphous carbon film, so that an extremely high adhesion is achieved.
In the present invention, the thickness of the interlayer is preferably 0.5 nm or over and less than 10 nm. By making it thinner than the thickness of interlayers used in the prior art, it is possible to obtain a high adhesion that was impossible in the prior art. If the thickness of the interlayer is thinner than 0.5 nm, the interlayer cannot perform functions as the interlayer because it is difficult to form a continuous film that is uniform in thickness over the entire surface of the substrate. If the thickness of the interlayer is over 10 nm, no sufficient adhesion is obtainable because the adhesion at the interface between the substrate and the interlayer or at the interface between the interlayer and the amorphous carbon film decreases. More preferably, the thickness of the interlayer is 2 nm or over and 7 nm or under.
As a method of forming the interlayer, a known method can be used such as vacuum deposition, sputter deposition, vacuum arc deposition, ion plating or various CVD. Among them, ion plating, sputter deposition and vacuum arc deposition are especially preferable because of high ionization rate of the raw material, and because due to the effect of driving ions into the substrate, a high adhesion between the interlayer and the substrate is obtained.
If the interlayer is formed on the substrate after contamination and an oxide layer on the substrate surface have been removed by irradiating the substrate surface with ions, a higher adhesion is obtained. Thus doing so is preferable. As a method of cleaning the substrate surface by ion irradiation, a known technique may be used.
According to the method of ion irradiation to the substrate surface, it is possible to simultaneously carry out the cleaning of the substrate surface by etching and the formation of the interlayer. Since a DLC film formed on the interlayer by this method is especially superior in adhesion, it is preferable.
Ion irradiation is carried out by applying a negative bias voltage to the substrate at least in the presence of ions of elements forming the interlayer. As a method of producing ions, a known technique may be used. But the use of a sputter evaporation source or a vacuum arc evaporation source is desirable because of high ionization rate and a fast etching speed.
In this case, the element forming the interlayer is used as a target. For example, if such a metal as Ti, Cr and Si is used for the interlayer, these metals can be used for targets. If a metallic carbide is used for the interlayer, the metallic carbide may be used for targets. If a metal is used for the target, hydrocarbon gas such as CH4 is supplied into the chamber as a carbon source, or using the metallic target, a metallic layer is formed on the substrate surface during ion irradiation and the metallic layer is carbonized during formation of the amorphous carbon film as described below to form a metallic carbide layer.
In order to simultaneously carry out etching of the substrate surface and formation of the interlayer, it is necessary to suitably select the value of the negative bias voltage applied to the substrate and the pressure of atmosphere. Although these conditions vary according to the ion irradiation method, if e.g. a vacuum arc deposition source is used, the negative bias voltage applied to the substrate should be xe2x88x92300V or over and xe2x88x921500V or under and the pressure of atmosphere should be 0.133 Pa or under.
During formation of the amorphous carbon film, or at least during the initial period of its formation, an interlayer of a metallic carbide can be formed by irradiating the surface of the metallic interlayer with high-energy carbon ions to carbonize the metallic interlayer. Although these conditions vary with the method of forming the amorphous carbon film, if vacuum arc deposition is used, the negative bias voltage applied to the substrate should be xe2x88x9250V or over and the pressure of atmosphere should be 0.7 Pa or under.
It can be confirmed whether or not the ion irradiation treatment is under the conditions for forming the interlayer simultaneously with the etching of the substrate surface, by confirming that the substrate has been etched after only ion irradiation has been carried out for a long time under the conditions and that an interlayer having a composition and thickness within the range of the present invention has been formed at the interface of the substrate and the amorphous carbon film, by the evaluation of cross sections of specimens on which an amorphous carbon film has been formed, by a transmission electron microscope or by the composition analysis in a depth direction using an X-ray photoelectron spectroscopy or an Auger electron spectroscopy.
In the present invention, a ceramics layer may be formed on the substrate as described below. In this case, too, the interlayer can be formed on the ceramics layer on the substrate by a method similar to the one mentioned above.
As a method of forming an amorphous carbon film, any known method may be used such as plasma CVD, sputter deposition, ion plating or vacuum arc deposition. Among them, sputter deposition and vacuum arc deposition are especially preferable because with these methods, an amorphous carbon film can be formed which is suitable for application to machine parts, molds, cutting tools, etc. because of good wear resistance and high hardness. Also, because these methods are high in the ionization rate of the carbon material and form a film under a relatively low atmospheric pressure, the energy of ions of carbon material that reach the substrate is so high that due to the effect of driving ions into the interlayer, an amorphous carbon film can be formed which has a high adhesive force.
In the present invention, it is necessary to carry out continuously ion irradiation treatment, formation of the interlayer, and formation of the amorphous carbon film in the same vacuum chamber, or to provide vacuum feed paths between a vacuum chamber for ion irradiation, a vacuum chamber for forming the interlayer, and a vacuum chamber for forming the amorphous carbon film, thereby treating continuously in vacuum. This is because if the ion-irradiated substrate is exposed to the atmosphere before the formation of the interlayer, contamination due to oxidation of the substrate surface or interlayer surface or adsorption of molecules develops, so that the effect of ion irradiation or formation of the interlayer would be lost.
Preferably, the amorphous carbon film of the present invention has a Knoop hardness (Hv) of 1200 or over and 8000 or under. If lower than 1200, the wear resistance would be so low that application would be limited. If higher than 8000, the internal stress of the film would be too high, so that the film tends to peel off.
Hardness may be measured by a push-in arrangement. Using a diamond Knoop indenter, with the load set at 50 g and the loading time at 10 seconds, the average value of measured values at ten points was obtained. If the shape of dents is difficult to see because protrusions and recesses on the film surface are large, buff polishing may be carried out with #8000 diamond paste for easy observation of the shape of the dents.
The thickness of the amorphous carbon film is preferably 0.05 xcexcm or over and 10 xcexcm or under. If thinner than 0.05 xcexcm, it would not exhibit properties of amorphous carbon itself such as low friction coefficient and high hardness. If thicker than 10 xcexcm, the surface roughness of the film would be too rough, so that the friction coefficient tends to increase or the film is liable to peel. Thus it is not suitable for practical use.
Since the amorphous carbon covered member of the present invention is characterized by high adhesion to the substrate, it is suited for applications in which durability is required at high loads of 9.8 Mpa or over such as cutting tools, molds and machine parts. It is not used in applications in which the load range is at a light load such as for magnetic recording media. As machine parts, it is especially suitable for parts for which a low friction coefficient and high durability are required, such as valve-train parts such as cams and valve lifters in internal combustion engines.
The present invention includes a structure in which a ceramics layer is formed between the substrate and the interlayer. This structure is especially effective in applications in which wear resistance is particularly required such as for cutting tools and molds. As the ceramics layer, it is possible to use a nitride of at least one element selected from elements in the IVa, Va, VIa and IIIb groups in the periodic table such as TiN, ZrN, VN, CrN, AIN or TiAlN, a carbide of at least one element selected from elements in the IVa, Va, VIa and IIIb groups in the periodic table such as TiC, or a carbonitride of at least one element selected from elements in the IVa, Va, VIa and IIIb groups in the periodic table such as TiCN.
Since these substances are superior in wear resistance, by using a structure in which one of these substances is disposed between the substrate and the interlayer, an amorphous carbon covered member can be provided which has superior wear resistance besides low friction coefficient, high weld resistance and high seizure resistance of the amorphous carbon film. If an amorphous carbon film is directly formed on such a ceramics layer or if an interlayer employed in the prior art is used, no sufficient adhesion is obtained. But by using the interlayer according to the present invention, it is possible to obtain an extremely high adhesion even on such a ceramics layer.
The ceramics layer may be either a single-layer film of one substance selected from the abovementioned substances, or a laminated structure in which two or more substances are laminated in a plurality of layers.
As the materials used for the ceramics layer, among the above-described ones, TiAlN, ZrN or VN is especially preferable because they are especially superior in the adhesion to the interlayer. If the ceramics layer has a laminated structure, these substances are preferably used for the uppermost layer of the ceramics layer because an especially excellent adhesion is obtained.
The thickness of the ceramics layer is preferably 0.2 xcexcm or over and less than 5 xcexcm. If less than 0.2 xcexcm, the wear resistance would not be improved. Over this range, the ceramics layer tends to peel off the substrate. More preferably, the thickness of the ceramics layer is 0.5 xcexcm or over and less than 3 xcexcm.
As a method of forming the ceramics layer, any known method such as plasma CVD, sputter deposition, ion plating or vacuum arc deposition may be used.
As the material of the substrate used in the present invention, any material may be used. But at least one selected from ceramics, iron-family alloys, aluminum alloys and iron-family sintered materials is preferable. As ceramics, silicon nitride, aluminum nitride, alumina, zirconia, silicon carbide, etc. can be cited. As iron-family alloys, high-speed steel, stainless steel, and SKD can be cited. As aluminum alloys, duralumin can be cited. Further, a cemented carbide of a tungsten carbide family metal, a diamond sintered material or a cubic boron nitride sintered material may be used according to the intended use.
The amorphous carbon covered member according to the present invention can be used for cutting tools, molds, machine parts, etc.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: